The invention relates to a two step crystallization process for recovery of a purified solvent from a liquid mixture and to an apparatus for carrying out the method. The invention in particular relates to a process where a) this starting mixture is subjected to a crystallization step where the solvent forms a suspension of solvent crystals and liquid residue and b) where these crystals are separated in wash columns to form pure solvent product and liquid residue that is passed to c) the second step where more solvent crystals are formed and d) these crystals are partially separated to form a solvent enriched product that is combined with the starting mixture in step a) and a liquid residue that is discharged from the process
The crystallization process can produce pure product crystals. Eutectic systems offer the possibility to produce pure product from mixtures with a wide range of impurities. The phase diagram, such as shown in FIG. 1, illustrates this for a typical chemical eutectic solution. Product B can be produced as pure crystal plus a liquid residue containing B plus impurities all the way down to the eutectic concentration (xe as wt % of B in liquid residue). The maximum recovery possible using only crystallization for a particular product is determined by the eutectic concentration xe. The equilibrium temperature (Te) of the solution at eutectic concentration is dependent on the physical properties of the specific product. It is important to note that this concentration is generally not attainable using a single step system. The limitations of the wash column in this aspect are further explained in U.S. Pat. No. 5,062,862. The production of an ultra-pure product by this crystallization technology requires the complete separation of the pure product crystal from the impurities remaining in the liquid residue.
The large crystal surface area that allows the optimum growth conditions for the production of pure crystals will negatively affect most attempts at separation. The impurities in the remaining liquid will adhere to all surfaces and complete removal of these impurities is necessary to finish the purification. Wash columns are perfectly suited to complete this task. The advantages of wash column use in similar applications are explained in detail in U.S. Pat. No. 5,062,862. Wash columns do, however, have certain restrictions with respect to their range of operation. When the requirement for ultra pure product is coupled with the requisite for high recovery rates, single stage crystallization would lead to an excessive concentration and temperature jump over the wash column. This is a major limitation of such a separation unit as also indicated in U.S. Pat. No. 5,062,862.
One solution to this problem was presented in the application of multiple stage crystallization as described in U.S. Pat. No. 4,787,985. For applications with a low reject concentration (low eutectic point and high recovery requirements) the installation of up to 5 stages is necessary in order to supply the wash column with a good conditioned feed. Technically the multistage crystallization is an attractive solution as it provides crystal growth at the best possible conditions. Also energy-wise it can offer advantages for systems with a low eutectic temperature, since only the last stage has to be operated at the low temperatures typically found at the eutectic whereas the purification stage feeding the wash column would operate at higher temperatures. The major disadvantage of this process is it's high investment cost.
Another idea to overcome the problem of a limited operating window for a packed bed wash column is described in the invention according to U.S. Pat. No. 5,062,862. Here the low temperature crystal slurry, after an initial partial separation of the crystals in a first wash column, is then reslurried in molten product and this suspension is then separated in a second wash column. The above stated limitations based on temperature jump over the first wash column is of no concern in this configuration since it is not attempted to provide a complete separation of crystals from the liquid residue in this first wash column; a certain carryover of liquid impurities after the first wash column is accepted because the final purification is performed in a subsequent second wash column.
A disadvantage of this reslurry process is the fact that all crystals are formed at the lowest temperatures and thus low concentrations of the main component as dictated by the recovery requirements. The crystal growth rate strongly decreases and the nucleation rate strongly increases when the concentration of the main component is reduced. Both effects result in the production of small crystals. To produce crystals of at least 100–200 μm, which are suitable for subsequent separation in wash columns, large residence times are necessary. This results in the requirement of large crystallizers with the corresponding high investment cost. For some applications it is not possible at all to grow crystals to the required size at those low temperatures. Small crystals would also lead to a more difficult solid-liquid separation—even if the required purities can still be achieved, the throughput of a wash column will decrease.
The object of the invention is to provide a process for the production of a crystal suspension under certain boundary conditions which process does not have the disadvantages above, or at least in a substantially reduced rate.
The present invention relates to a two-step crystallization purification process for the recovery of a high purity end product. As indicated earlier the present crystallization processes are disadvantaged by the high capital cost and relatively complicated equipment. The invention reduces the number and complexity of the required components and therefore reduces the investment costs. It has now been discovered that various chemical mixtures do not require the installation of multiple stages within each section. The type of crystallizer can also be simplified and the recrystallization vessel is omitted. These finding can significantly reduce the investment and operation costs when compared to the process described in U.S. Pat. No. 4,787,985.